Yes, I too would like to learn more.
A couple things to keep in mind: National Guard are not routinely armed when in and among the populace. And when armed, they are armed only to protect themselves. That is, they cannot act as law enforcement or general troops. Their rules of engagement are purely defensive. So the National Guard being "disarmed" would be the rule, not the exception. It would depend here upon the exact circumstances, orders, and rules of engagement.
As far as the National Guard presence in DC, it's easy to conflate the two purposes immediately at hand. On the one hand there is an extraordinary presence of Guards in the Capitol, at other places, and immediately outside the District. This is in natural response to the Capitol attack. But for the typical inauguration, each state sends a a company or so of its National Guard to attend the inauguration and provide ordinary levels of security such as for crowd control, traffic direction, etc. This is mostly ceremonial and it's very uncommon for those honor guards to be readily armed. But of course this inauguration is special, so there may be special guidelines or orders. Yes, there was special screening. My inside source is my brother-in-law, who is a lieutenant in the Utah National Guard. No, they're not asking who you voted for. They're repeating the part of the ordinary initial screening for Guard troops to weed out extremists. (If you excluded all Republican-voting members of our National Guard, you'd have maybe one platoon left.)
Now as for Marines in DC, there are always Marines in DC. There has been a company of Marines stationed there since the Jefferson Administration. And part of their mission has always been to guard parts of the capitol city as needed. The White House, for example, is always guarded by Marines, usually in highly ceremonial costume, but not just as a show. So if they are deployed now, it would be entirely normal. If there are additional Marines in DC beyond the normal complement, that's something else. And if they are armed, that too would be something else. Again, there is a deep tradition in the U.S. against deploying active-duty military as a domestic police force. "Full combat gear" may include weapons but, for example, not ammunition. And combat readiness may be indicated simply because it includes helmets and body armor -- clearly indicated following the Jan. 6 incident.